1. Field of the Invention
The present invention relates in general to a thermoelectric device for creating a martensitic state in shape memory alloy, and in particular to a hand held device to be used in orthodontic procedures. Still more particularly, the present invention relates to a thermoelectric cooling device that is continuously gas or liquid cooled to allow continuous use at a constant cooling temperature in orthodontic procedures.
2. Description of the Prior Art
Shape memory alloys have a variety of practical uses, one of which is in orthodontic and other medical procedures. One example of the use of shape memory alloy is the Sachdeva et al. (U.S. Pat. No. 5,044,947)invention of an Orthodontic Archwire and Method of Moving Teeth. In that invention, a nickel-titanium-copper alloy is formed into an arch-shaped wire that holds its shape. The room temperature, rigid state of this alloy is called its austenitic state. However, when a localized portion of the wire is cooled, the metal is placed in a martensitic state. The actual temperature required to bring the wire to its martensitic state may range from about 15.degree. C. to 0 .degree. C., depending upon the type of wire used. In the martensitic state, the wire becomes soft and pliable in the cooled region. This characteristic is useful in applying the archwire to brackets placed on teeth. Cooling the wire allows easy placement of the wire on a maloccluded tooth, and upon warming to room or body temperature, the archwire tends to go back into its arch-shape, thus correcting the malocclusion.
The problem with using shape memory alloys such as that described in Sachdeva et al. is in conveniently bringing a localized portion of the wire to its martensitic state. Some of the devices and methods in current use by orthodontic practitioners is the use of frozen cotton swabs, placing the wire in a freezer or cooler, or the use of a freezing instrument, an example of which goes by the trade name of POLAR BEARTM.TM. (Orec Corporation). The latter device is a hollow pencil-shaped device that is filled with water, then frozen. There are several problems with these devices. Once removed from the freezer, the devices must be used immediately. Further, the practitioner must have a freezer close to the patient for cooling the devices. Also, the cotton swab(s) or POLAR BEAR.TM. do not stay cold enough for a substantial amount of time to be useful for procedures longer than 1-2 minutes without further, time consuming freezing in a freezer.
There are several medical instruments that are hand held cooling devices incorporating a narrow tip for applying to the desired location to be cooled. The Crump et al. (U.S. Pat. No. 3,575,176) invention is a cryosurgical instrument that has a cooling tip, the tip cooled by a refrigerant such as dichlorodifluoromethane or carbon dioxide charged into the device. The device is used mostly in cataract surgery, being used to adhere the cooling tip (which is at -10 to -20.degree. C.) to the tissue to be extracted or handled. The Lisenbee et al. (U.S. Pat. No. 3,993,075) invention is similar to the Crump et al. invention, with the added feature of having a rapid defrosting means. These devices are cumbersome, difficult to use, and use undesirable chemical compounds that escape into the air. Further, they tend to cool the region more than is needed for placing a shape memory alloy in its martensitic state, and may even injure sensitive gum and oral tissues.
Thermoelectric technology, a common technology incorporating the Peltier Effect, would be ideal for use in various medical and orthodontic practices. Thermoelectric devices can be miniaturized to fit into small, hand held instruments, and require little to no maintenance when proper cooling means are provided. There is one hand held device that incorporates thermoelectric technology used for testing the sensitivity of a patient's tooth in order to determine if the tooth is alive or dead, the Tamler et al. (U.S. Pat. No. 4,308,012) invention. This device has a cooling tip that is used to contact the patient's tooth, applying either heat or cold to the tooth, the practitioner then looking for a response from the patient to determine the sensitivity of the tooth. The device has precise and complex electronic circuitry to allow control of the cooling tip temperature and provides an automatic cut-off if the apparatus overheats.
There are several problems with the Tamler et al. device. The first and most inhibiting problem is that it can overheat after prolonged (greater than 1-2 minutes) use, causing the device to automatically cut-off. This makes the device impractical to use in orthodontic procedures, which can last 1-2 hours. Also, the precise circuitry involved increases the cost and maintenance of the device. Finally, the Tamler et al. device does not allow for easy changing out of the cooling tip, which would be useful for various applications as well as for hygienic purposes. Thus, there is a need for a device that is less expensive, easy to use and maintain, and can be used continuously for at least 1 hour. The present invention solves these problems.